Recovery of aroma gases

ABSTRACT

Coffee aroma gases are conveyed to a cryogenic collector in which liquid nitrogen is sprayed directly into the aroma gas stream to rapidly condense the aroma gas and form finely divided particles of coffee aroma frost suspended in a stream of nitrogen gas, while minimizing contact of the cooled gas with the walls of the collector. The suspension of aroma frost particles in gaseous nitrogen is passed through a tubular porous filter to remove the aroma frost particles which collect on the outer surface of the tubular filter, with the nitrogen gas passing through the porous filter and being exhausted from the collector. The porous filter is periodically back pulsed to dislodge aroma frost particles collected on the outer surface of the tubular filter, with the particles being recovered for incorporation in soluble coffee products. Condensation of aroma components present in the aroma gas may be carried out in several stages to effect fractionation of the aroma gas.

This application is a continuation-in-part of pending application Ser. No. 07/980,025, filed Nov. 23, 1992, now U.S. Pat. No. 5,222,364 which is a divisional application of application Ser. No. 07/760,662, now U.S. Pat. No. 5,182,926.

This invention relates to the recovery of volatile aroma gases. More particularly, the invention relates to the low temperature condensation and recovery of volatile aroma constituents from gases evolved during the processing and recovery of a beverage material, particularly during the processing of coffee, and to the fractionation of the aroma constituents of such gases.

BACKGROUND OF THE INVENTION

Gases containing volatile aroma constituents are evolved during one or more stages of processing beverages such as coffee, tea, and cocoa. For example, in coffee processing, gases containing aroma constituents are given off during a number of stages of the processing operation, such as during roasting of whole beans, grinding of the roasted whole beans, extraction of water solubles, steam distillation and the like. It has been recognized that the aroma and flavor of such beverage products, particularly soluble coffee products, can be improved by recovering the aroma gases and incorporating them in the beverage product, such as by addition of the aroma constituents to coffee extract prior to drying or to the dry powder or granules. Frequently coffee aroma gases are recovered by low temperature condensation of the gases at liquid nitrogen temperatures to form a coffee aroma frost. A number of procedures have been suggested heretofor for the recovery of coffee aroma frost. One such procedure which is widely used involves condensing the aroma gases on the walls of a scraped-surface heat exchanger which is cooled by means of liquid nitrogen. The condensed gases are scraped from the walls and collected at the bottom of the heat exchanger in the form of a frost or snow which is recovered. However, this procedure is inefficient and is subject to a number of disadvantages. For example, the total gas stream entering the heat exchanger is not necessarily entirely cooled down and fine frost particles tend to become entrained in the gas stream and leave the heat exchanger without being caught.

SUMMARY OF THE INVENTION

The present invention provides a simple but highly effective and efficient procedure for the collection and recovery of aroma frost from gases evolved during processing of beverage products particularly coffee. The invention will be particularly described with respect to aroma gases evolved during coffee processing; however, it is to be understood that other aroma-bearing gases such as those evolved in the manufacture of tea and cocoa, may likewise be employed and are considered to be within the scope of this invention. In the present invention, coffee aroma gases, such as gases evolved during grinding of roasted whole beans, are conveyed to a cryogenic collector in which a cryogenic liquid, preferably liquid nitrogen, or a cryogenic gas, preferably cold nitrogen gas, is injected into the aroma gas stream to rapidly cool the aroma gas and effect rapid condensation of the aromatic constituents of the gas. Direct contact of the cryogenically cooled aroma gas stream with the walls of the collector is minimized, preferably being avoided. Such cryogenic cooling of the aroma gas stream takes place upon contact of the aroma gas stream with the cryogenic liquid or gas, and results in the formation of fine particles of aroma frost suspended in cryogenic gas, such as gaseous nitrogen produced by the evaporation of the liquid nitrogen spray. The suspension thus formed of aroma frost particles in the cryogenic gas stream, is passed through one or more porous filters mounted in the cryogenic collector to remove the aroma frost particles from the gas stream. Preferably the porous filters comprise porous cylindrical tubes, closed at the bottom, with the gas phase of the gas suspension passing through the walls of the cylindrical filters. The aroma frost particles are retained on the outer or upstream walls of the porous tubular filters and form a filter cake on the outer walls. The gas phase of the suspension passes through the walls of the tubular filter by maintaining a sufficient pressure differential across the walls, with the gas phase passing to the interior of the tubular filters and then being exhausted through the top of the filters to a point outside the collector. The porous filter is periodically back pulsed with nitrogen or other inert gas to dislodge the aroma frost particles from the surface of the filter, with the frost particles falling to the bottom of the collector from where the particles are removed and recovered either on a continuous or a periodic basis.

Preferably, the cold nitrogen gas passing through the porous filter is vented into an insulated housing which surrounds the cryogenic collector thereby maintaining the collector and porous filters at temperatures sufficiently low to maintain the aromatic constituents of the aroma gas in condensed form.

In accordance with an embodiment of this invention, condensation of the aroma components present in the aroma gases may be carried out in several stages to effect fractionation of the aroma gas. In this embodiment, several (typically 3 to 5) of the cryogenic collectors described above may be connected in series, with successive collectors being operated at successively lower temperatures. Each of the cryogenic collectors is provided with control means to maintain the temperature in each controller within a predetermined range, preferably ±5° C. from a predetermined temperature while maintaining a constant supply of liquid nitrogen to the collector. The aroma condensate fractions recovered at the various temperature levels may be used for aromatizing soluble coffee products or may be discarded, depending upon the aroma characteristics of the particular aroma fraction.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view of an aroma frost recovery unit in accordance with one embodiment the present invention.

FIG. 2 is a side plan view, partly cut away, of the upper portion of another embodiment of the aroma frost recovery unit of this invention.

FIG. 3 is a diagrammatic vertical sectional view of the cryogenic collector in which aroma gases are condensed with a cryogenic gas.

FIG. 4 is a diagrammatic flow sheet which shows the use of several cryogenic collectors in series to fractionate an aroma gas.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be particularly described with respect to the recovery of aroma gases evolved during coffee processing. It will be understood, however, that the present invention is also applicable to the recovery of other aroma-bearing gases, such as those generated in processing tea and cocoa.

The aroma gas evolved at any one of several points in the processing of coffee may be used in the present invention. For example, gases evolved during roasting of green coffee (i.e. "roaster gases"), gases evolved during grinding of roasted whole beans (i.e. "grinder gases"), and those evolved during infusion or water extraction of ground roasted coffee (i.e. "infusion gases") may be used. Typically the composition of the aroma gases evolved is largely (i.e. up to 90% or more by weight) carbon dioxide together with water vapor and the characteristic aromatic constituents of roasted coffee. If desired, the amount of moisture in the gas may be reduced by passing the evolved gas through a condenser where it is cooled to a temperature between about -25° C. and 10° C. in order to remove substantial quantities of water from the gas stream. Where desired, a stream of inert gas, such as nitrogen or carbon dioxide, may be used to sweep the evolved aroma gas from the coffee and carry it through the aroma recovery operation. The aroma gas stream is normally maintained under a pressure sufficient to move the gas through the recovery operation, typically 0.5 to 5 psig, taking into account the pressure drop across the filter units employed. Alternatively, the aroma gases may be pulled from the aroma generating equipment and carried through the aroma recovery operation by means of a vacuum. The use of a vacuum to move the aroma gases eliminates the need for a sweep gas thereby increasing the concentration of the aromatic constituents in the gas stream and increasing the efficiency for condensing the aromatic constituents.

The invention will be primarily described in regards to the use of liquid nitrogen to condense the aroma gas. However, as previously mentioned, other cryogenic liquids as well as cryogenic gas, such as cold nitrogen gas, which are capable of effecting rapid condensation of the aromatic constituents of the aroma gas, may be used. In accordance with the present invention, the coffee aroma gas is carried via suitable piping to a cryogenic collector and liquid nitrogen is sprayed into the aroma gas stream to rapidly cool the gas to a temperature at which finely divided aroma frost particles are condensed with the aroma frost particles being suspended in nitrogen gas produced by evaporation of the liquid nitrogen with which the aroma gas is contacted. Preferably, the amount of liquid nitrogen sprayed into the aroma gas stream is controlled by use of suitable valve means to cool the gas to a temperature of at least about -80° C. or below, with a temperature of between about -100° C. and -160° C. being more desirable, with the collector being maintained at similar cryogenic temperature levels. Such temperatures serve to effect rapid condensation of the aromatic constituents of the gas stream and maintain the aroma frost well below the boiling point of the most volatile components and below the sublimation point of carbon dioxide, while ensuring that substantially all of the liquid nitrogen sprayed into the gas stream evaporates.

The aroma gas and liquid nitrogen are introduced into the cryogenic collector in a manner which facilitates rapid, substantially uniform cooling of the gas while minimizing or avoiding contact between the cold gas and the walls of the collector. For example, the aroma gas stream may be introduced into the cryogenic collector by means of an inlet pipe which discharges the aroma gas into the center of the collector. In accordance with one embodiment, a plurality of liquid nitrogen spray nozzles are mounted in the collector adjacent the discharge of the aroma gas inlet pipe, and are oriented so that their spray patterns converge at the midpoint of the aroma gas discharge and induct the aroma gas flow into the center portion of the collector. According to another embodiment the liquid nitrogen spray nozzle is mounted coaxially within the aroma gas inlet pipe adjacent the discharge end of the pipe whereby liquid nitrogen is sprayed into the aroma gas in the inlet pipe to produce a suspension of condensed aroma frost particles in nitrogen gas which is carried into the collector.

The suspension of aroma frost particles in nitrogen gas thus formed is directed through one or more rigid porous filters mounted in the cryogenic collector to remove substantially all of the finely divided frost particles from the nitrogen gas. Preferably, the porous filters comprise porous tubes, closed at the bottom and having an open upper end, with the gas phase of the suspension passing through the walls of the tubular filter from the outer or upstream surface to the inner or downstream surface of the filter and being discharged through the open upper end. The porous tubular filters may have any suitable cross-sectional configuration such as circular, oval, rectangular, star-shaped, and the like. Advantageously, a venturi nozzle is secured over the open upper end of the filter, with the nitrogen gas being exhausted through the venturi.

The porous cylindrical filters used in the present invention have a pore size distribution such that substantially all of the aroma frost particles are removed from the nitrogen gas suspension passing through the filter, with the filters being thermally compatible with the cryogenic conditions maintained in the collector. In addition, the porous filters have sufficient structural strength and durability to withstand cyclic pressure increases during reverse flow cleaning cycles. Filters made of porous ceramic or porous metal such as porous stainless steel, sintered woven wire mesh, and the like are suitable for use in the present invention. Cylindrical filters made of PSS® porous stainless steel, Grade H, sold by Pall Porous Metal Filters Corporation, Cortland, N.Y., which has an absolute rating of approximately five microns, have been found to be effective in removing over 95% of the aroma frost particles from the gaseous suspension.

The size and number of filters mounted in the collector depends to a large extent on the flow rate of the aroma gas being processed by the cryogenic collector, and can be readily determined by routine experimentation. For example, a single porous stainless steel cylindrical filter 6.1 cm diameter and 50.8 cm in length, having about 960 sq. cm. of filter area and an average pore size of 20 micrometers is effective in removing substantially all of the frost particles from a coffee aroma gas stream having a flow rate of approximately 200 SCFH.

The gas phase of the suspension is forced through the pores of the porous cylindrical filter by maintaining a sufficient pressure differential across the walls of the filter, either by introducing the aroma gas into the collector under pressure or by maintaining the collector under slight vacuum. The gas phase passes through the porous walls to the interior of the cylindrical filter and is exhausted from the cryogenic collector through the open upper end of the filter. This gas phase which has a temperature of about -140° C., may be exhausted to the atmosphere, but preferably is vented into an insulated housing which surrounds the cryogenic collector. Venting the nitrogen gas exhaust stream in this manner is effective in maintaining the temperature of the cryogenic collector sufficiently low (i.e. about -115° C.) to maintain the aroma frost particles in condensed form.

During filtering, aroma frost particles, which are removed from the nitrogen gas suspension passed through the filter, build up and tend to form a filter cake on the outer walls of the cylindrical filter. The accumulating cake of aroma frost particles is dislodged and removed from the filter by periodically directing a pulse of gas, preferably gaseous nitrogen, into the open upper end of the cylindrical filter, at a pressure appreciably greater than on the outer wall of the filter. The pulses are of 0.1 to 1.0 second duration with the pulses being repeated at a time interval in the range of 1 to 3 minutes. Typically nitrogen gas at a pressure of approximately 90 psig is used.

The aroma frost particles dislodged from the filters fall to the bottom of cryogenic collector and are removed either periodically or on a continuous basis. For example conventional means such as a rotary valve, a screw conveyor, and endless belt, and the like may be provided at the bottom of the collector for the removal of the dislodged aroma frost particles from the collector on a continuous basis. If desired a suitable liquid such as coffee oil, or a bland tasting vegetable oil such as cottonseed oil or coconut oil may be pumped through the base of the cryogenic collector, with the aroma frost particles forming an emulsion in the liquid carrier. Alternatively, an insulated container may be removably secured to the bottom of the collector below the filters, with the dislodged frost particles falling into the container, which is removed periodically for recovery of the aroma frost particles.

The aroma frost thus recovered is effective in improving the aroma and flavor of beverages, particularly soluble coffee. For example, coffee aroma frost particles recovered in accordance with the present invention may be added to jars of soluble coffee powder to aromatize jar head space and increase the flavor and aroma of the coffee. Alternatively the aroma frost particles may be added to coffee oil and used to aromatize jar head space. In addition, the coffee aroma frost particles may also be added to liquid coffee extract which is then dried, in order to produce an improved cup aroma when the powder is dissolved in hot water.

Referring now to FIG. 1 of the drawings, the aroma frost recovery unit of the present invention includes a cryogenic collector 10 mounted centrally within an insulated cylindrical housing 11 by means of support brackets 12. Cryogenic collector 10 comprises a cylindrical shell 14 closed at the top by wall 15 and having a funnel-shaped bottom section 16 with an orifice 17 through which condensed aroma frost may be removed. Insulated container 18 is removably secured to bottom section 16 by suitable clamping means 19, such as a Tri-clamp fitting. Container 18 is adapted to be removed periodically through a door (not shown) in the base of housing 11.

Tubular porous filters 20, of suitable cryogenic filtering material, for example, porous stainless steel are mounted within collector 10 adjacent the top wall 15 thereof. Filters 20 are closed at the bottom end such as by welded end cap 28 and have an open upper end, with a venturi nozzle 21 being secured over the open upper end of each tubular filter 20. Each of the venturi nozzles 21 extends through a suitable opening in top wall 15 of collector 10 to exhaust gas from the interior of a tubular filter 20 out of cryogenic collector 10 into cylindrical housing 11 from which the gas is discharged through exhaust line 27. Inlet tube 22, which carries aroma gas evolved during coffee processing, communicates with the interior of collector 10 through opening 23 in the upper portion of shell 14. Inlet tube 22 is disposed normal to the sidewall of shell 14 at a slight downward angle so as to direct the flow of aroma gas discharged from tube 22 toward the vertical midline of shell 14, away from the sidewalls. Liquid nitrogen from supply tank 25 is carried in line 24 into collector 10 with the liquid nitrogen being sprayed from nozzles 26 mounted in collector 10 adjacent the discharge end of inlet tube 22. Spray nozzles 26 are oriented so that their spray patterns are directed away from the sidewalls of shell 14, toward the center of the aroma gas stream discharged through opening 23. A suitable nitrogen vent valve 28 is provided in line 24 to control the temperature within the cryogenic collector 10 by increasing or decreasing the flow of liquid nitrogen to spray nozzles 26.

Pressurized nitrogen gas is carried in line 30 to pulsing tank 31, with a periodic back-pulse of nitrogen gas being introduced into the interior of tubular filters 20 through pulse lines 32 which extend, coaxially into each of the venturi nozzles 21 mounted on tubular filters 20. Any of the pulse generating means commonly used to generate pulses of pressurized back flushing gas may be used in the present invention.

FIG. 2 shows a modified arrangement for introducing the aroma gas stream and the liquid nitrogen into the cryogenic collector. In this embodiment, cryogenic collector 10 includes a cylindrical housing 40 which is mounted on top wall 15, with the interior of housing 40 being in communication with the interior of collector 10 through central opening 41 in the top wall. Aroma gas inlet pipe 22 extends through the top of housing 40 at the center thereof so as to discharge a stream of aroma gas vertically downward in housing 40 and shell 14. Liquid nitrogen is carried to the collector in lines 24, with the liquid nitrogen being sprayed into the aroma gas stream through a plurality of nozzles (not shown) spaced equidistant around the housing adjacent the discharge end of inlet pipe 22. The spray nozzles are oriented so that their spray patterns converge at the center of the housing a short distance below the discharge end of inlet pipe 22, with the nozzles being pitched at a downward angle of about 30° so as to induct the aroma gas stream into the shell portion of collector 10. This arrangement is particularly effective in the uniform, rapid cooling of the aroma gas stream to cryogenic temperatures while minimizing contact between the cooled gas and the walls of the collector.

In operation, wherein the apparatus shown in FIG. 1 is used, coffee aroma gas comprising primarily carbon dioxide together with small amounts of water vapor and coffee aroma constituents, which is evolved during coffee processing operations, is carried in tube 22. The aroma gas, which typically is under a pressure of between about 0.5 to 5 psig, is discharged into cylindrical shell 14 in a downwardly direction, toward the center of the shell and away from the sidewalls thereof. Liquid nitrogen is sprayed into the aroma gas stream through nozzles 26 mounted within casing 14 equidistant around the discharge of tube 22 with the liquid nitrogen spray being directed toward the center of the collector so that the aroma gas stream discharged from tube 22 is rapidly and uniformly cooled to condense the aroma gas as finely divided frost particles suspended in a stream of nitrogen gas, while avoiding or minimizing contact of the cooled aroma gas stream with the sidewalls of shell 14. The supply of liquid nitrogen to nozzles 26 is adjusted to maintain the temperature in collector 10 below -80° C., usually between about -140° C. and -150° C. to maintain the aroma frost particles at a temperature below the boiling point of its most volatile constituents while ensuring that substantially all of the liquid nitrogen is volatilized. Three rigid tubular porous metal filters 20 are mounted in collector 10 adjacent its top wall, with each of the filters having a venturi nozzle 21 attached to the upper end of the filter. Each of the nozzles 21 extends through the top wall of the collector 10. The filters are made of porous stainless steel having a removal rating of 5 microns with the tubular filters having a length of about 50 cm and a diameter of about 6 cm. The nitrogen gas phase of the suspension is forced through the pores of the filter tubes by maintaining a pressure differential of about 25 to 50 cm of water across the walls of the filters, while the aroma frost particles are deposited on the outer surface of the filters. The nitrogen gas which is at a temperature of about -140° C. passes through the porous filter walls to the interior of the filters, through the venturi nozzle at the top of each filter, and is vented from collector 10 into the interior of insulated housing 11 surrounding shell 14, from where it is exhausted to the atmosphere through exhaust line 27. In this manner the temperature within the housing is maintained at about -120° C. or below. The tubular filters are back pulsed every 60 seconds for a period of 0.3 second with nitrogen gas at a pressure of about 90 psig to dislodge aroma frost particles which collect on the exterior of the filters, with the frost particles being collected in container 18. The aroma frost particles are periodically removed from container 18 for incorporation in liquid coffee extract or in soluble coffee particles to increase the roast and ground coffee aroma and flavor.

FIG. 3 illustrates an embodiment of the invention in which a cryogenic gas, preferably nitrogen gas at a temperature of about -100° C. or below, is introduced into the cryogenic collector to condense the aroma gas. As shown in FIG. 3, aroma gas is introduced into cryogenic collector 10 through line 51 and is contacted by cold nitrogen gas. The aroma gas is condensed into the form of frost particles as it is contacted with the cold nitrogen gas. For example, a gas flow ratio into the cryogenic collector of about 3-5 parts of nitrogen gas having a temperature of about -196° C. per part of aroma gas in a nitrogen sweep gas is effective to condense substantially all of the aroma gas constituents. The nitrogen gas used preferably is produced by evaporation of liquid nitrogen from a supply tank (not shown) into a closed chamber (not shown) and is introduced into cryogenic collector via line 52. Aroma gas inlet line 51 and nitrogen gas line 52 are arranged as described above so that the gas streams are directed away from the sidewalls of collector 10. A portion of the aroma frost particles condensed from the aroma gas fall to the bottom of collector 10, and a portion are suspended in the nitrogen gas which is passed through the pores of porous tubular filter 20 with the aroma frost particles being retained on the outer walls of the tubular filter. Nitrogen gas passing through the tubular filter is vented to outside the collector at 53. The aroma frost particles are dislodged from the tubular filter by a periodic back pulse of gas and fall to the bottom of the collector.

Aroma frost particles are removed from collector 10 through rotary valve 54 into container 55 which is secured to the base of cryogenic collector 10. The aroma frost particles are combined with a liquid carrier 56, such as concentrated coffee extract, an edible oil or an emulsion of edible oil in coffee extract, which is introduced into container 55 through inlet line 57. The recovered aromas typically are warmed to above the freezing point of the carrier liquid. Upon warming, carbon dioxide condensed with the aroma gases vaporizes and is discharged through line 59. The recovered aromas in the carrier liquid are removed from container 55 through line 58.

In an alternative mode of operation, the recovery of aroma gas can be carried out in several stages to effect fractionation of the aromatic constituents of the gas. That is, the aroma-bearing gas stream is passed through a number of cryogenic collectors, such as that shown in FIG. 1, connected in series, with each succeeding collector being operated at a lower temperature than the one preceding it. In this manner a series of aromatic constituents having successively lower freezing points are condensed out of the aroma gas stream and recovered. Each of the series of cryogenic collectors is provided with temperature control means to maintain the temperature in each collector within a predetermined range. Suitable temperature control means include, for example, a temperature controller associated with each collector for determining the temperature within the collector such as by a suitable thermocouple mounted within the collector, and comparing it to a set point temperature. The temperature controller is also associated with and controls a vent valve of the liquid nitrogen supply tank to the collector. If the temperature in the collector is below the preset level, the vent value is opened further to allow more nitrogen gas to escape. This escape of gas reduces the pressure within the liquid nitrogen supply tank, thus reducing the pressure drop across the spray nozzle in the collector. By reducing the pressure drop across the nozzle, the flow of liquid nitrogen is reduced and the temperature within the collector rises. Similarly, if the temperature in the collector is too high, the vent valve is closed, which increases the flow of liquid nitrogen to the spray nozzle, thereby reducing the temperature in the collector. In addition to controlling the flow of liquid nitrogen to the cryogenic collector to control the temperature in the collector, control means are also provided to maintain a constant supply of liquid nitrogen to the nozzles in the collector.

This accurate control of temperature within the collectors enables a number of collectors to be connected in series and operated at successively lower temperatures, thereby effecting fractionation of the aromatic constituents of the aroma gas.

This embodiment is shown in FIG. 4 in which aroma gas, such as grinder gas resulting from grinding roasted whole coffee beans, in a nitrogen sweep gas, is passed through four cryogenic collectors in series which are operated at successively lower temperatures. Each of the cryogenic collectors 10 has the construction described in connection with FIGS. 1 and 3 and includes a porous tubular filter 20 having a pore size distribution which retains aroma frost particles on the outer side of the filter while permitting cryogenic gas to pass therethrough. The aroma gas carried in line 61 first passes through heat exchanger 62 which is cooled by chilled water to reduce the temperature of the aroma gas to about 1.6° C. Water vapor condenses from the gas stream in the heat exchanger, together with the least volatile or highest boiling aroma constituents, and is collected in container 63. This condensate is useful in enhancing the flavor of soluble coffee products.

Non-condensed aroma gases pass from heat exchanger to cryogenic collector A through line 64. The aroma gas stream in the collector is contacted by liquid nitrogen supplied through line 65. The temperature in collector A is maintained at about -18° C. ±5° C. by temperature controller 66 which controls the temperature within a desired range by controlling the flow of liquid nitrogen into the collector. That is, the temperature within the collector is determined by thermocouple 67 and compared to the set point temperature (-18° C.) for collector A, and a nitrogen gas vent valve (not shown), associated with nitrogen gas supply line 68, is either opened or closed as described hereinabove, to maintain the temperature in collector A within the desired range. Aroma gas constituents which condense at temperatures above -18° C. ±5° C. fall to the bottom of collector A and are recovered in container 80. The aroma frost particles which are condensed at a temperature of -18° C. ±5° C. are less volatile than those condensed in heat exchanger, have a stronger, sharper aroma, and are useful in enhancing the flavor of soluble coffee products.

Aroma gases which are not condensed at the temperature maintained in cryogenic collector A pass through porous tubular filter 20 and are introduced via line 70 into cryogenic collector B. Collector B is maintained by temperature controller 71 at a temperature of about -34° C. ±5° C., with the aroma gas being contacted by a liquid nitrogen spray to condense aroma frost particles. Aroma frost particles condensed at a temperature of between -18° C. and -34° C. are condensed in collector B and are recovered from container 81. The frost particles condensed in this temperature range have an objectionable sulfury, cabbage odor and are discarded.

Aroma gases which are not condensed at the temperature maintained in collector B are carried through line 72 into cryogenic collector C which is maintained by temperature controller 73 at a temperature of about -80° C. ±5° C. to condense carbon dioxide and aromatic components having a roasty aroma. The aroma frost particles condensed at this temperature are recovered from container 82 and are useful in providing either in-cup or in-jar aroma enhancement.

Aroma gases which are not condensed at a temperature of -80° C. ±5° C. are carried through line 74 into collector D which is maintained by temperature controller 75 at a temperature of about -157° C. ±5° C., with the aroma frost particles condensed at this temperature being recovered in container 83. These aroma frost particles are very volatile and have a sweet, buttery aroma. These aroma frost particles are particularly well suited for enhancing the in-jar aroma of soluble coffee products. 

What is claimed is:
 1. In a method of recovering aroma gas evolved during processing of a beverage material in which the aroma gas is conducted to a cryogenic collector, a cryogenic liquid or gas is injected into the aroma gas in said collector under conditions sufficient to condense aroma frost particles from the aroma gas to form a suspension of aroma frost particles in cryogenic gas, the gaseous suspension is contacted with a porous filter medium having a pore size distribution which removes substantially all of the aroma frost particles from the gaseous suspension with the particles being deposited on the filter medium and the cryogenic gas passing the porous filter medium, and the aroma frost particles are dislodged from the filter medium and recovered, the improvement which comprisespassing the aroma gas through a plurality of cryogenic collectors in series with the cryogenic collectors being maintained at successively lower temperatures, with the first collector being maintained at a temperature at which only a portion of the aroma constituents is condensed and the last collector in the series being maintained at a temperature at which substantially all of the aroma constituents remaining in the aroma gas are condensed, with intermediate cryogenic collectors being maintained at temperatures intermediate said first and last collectors, to thereby effect fractionation of the aroma constituents of the aroma gas, and controlling the flow of cryogenic liquid or gas to each cryogenic collector to maintain the temperature in the collectors at a predetermined level.
 2. The method defined in claim 1 in which the aroma gas is evolved during the processing of coffee.
 3. The method defined in claim 2 in which liquid nitrogen is injected into the aroma gas in each collector to condense aroma gas particles.
 4. The method defined in claim 3 in which the flow of liquid nitrogen into the first cryogenic collector is controlled to maintain the temperature in said collector at about -18° C. ±5° C. to condense a portion of the aroma constituents from the aroma gas, with the condensed particles being removed and the non-condensed aroma gas being introduced into a successive cryogenic collector maintained at a temperature lower than said first collector.
 5. The method defined in claim 4 in which the aroma gas is passed through four cryogenic collectors in series, with the temperature in the first, second, third and fourth collectors being maintained at -18° C. ±5° C., -34° C. ±5° C., -80° C. ±5° C., and -157° C.±5° C. respectively.
 6. The method defined in claim 5 in which the aroma frost particles condensed in the first, third and fourth collectors being recovered for use in aromatizing soluble coffee products, and the aroma frost particles condensed in the second collector being discarded.
 7. The method defined in claim 3 in which the temperature in the cryogenic collectors is maintained at a predetermined level bymeasuring the temperature within the collector, comparing the measured temperature with a predetermined desired temperature for the collector, and adjusting the temperature in the collector in response to the comparison of the measured and predetermined temperatures by adjusting the pressure drop across the discharge of liquid nitrogen into the collector, with the pressure drop being reduced in order to increase the measured temperature in the collector, and being increased in order to reduce the measured temperature.
 8. A method of recovering aroma gas evolved during processing of a beverage material which comprisesconducting the aroma gas to a cryogenic collector, contacting the aroma gas in said collector with a cryogenic gas under conditions sufficient to condense aroma frost particles from the aroma gas to thereby form a suspension of aroma frost particles in the cryogenic gas, contacting the gaseous suspension with a porous filter medium having a pore size distribution which removes substantially all of the aroma frost particles from the gaseous suspension, with the particles being deposited on the filter medium and the cryogenic gas passing through the porous filter medium, removing the aroma frost particles from the filter medium, and recovering the condensed aroma frost particles. 